Hubble Tuning Fork Diagram

Thanks to the University of Texas, McDonald Observatory for this diagram

The tuning fork diagram was devised by Hubble as a means of classifying galaxies by their appearance. The ellipticals are arranged in one branch while the spirals and the barred spirals form two parallel branches. The ellipticals are arranged in order of increasing ellipticity from left to right while the spirals and barred spirals become less tightly wound with smaller nuclei. The amount of interstellar dust in the galaxies increases from left to right in the diagram while the effect of rotation becomes more important.

There is a S0 galaxy at the junction of the three arms of the diagram. S0 galaxies are disc-shaped like spirals but without the spiral structure and interstellar gas. They look like flat ellipticals.

It was originally thought that galaxies evolve along the tuning fork diagram but this is now known not to be true. Instead their properties are determined at their birth.